A nuclear reactor plant includes a containment vessel surrounding a reactor pressure vessel and defining therewith a drywell which typically contains a non-condensable gas such as nitrogen. Disposed in the pressure vessel is a conventional nuclear reactor core submerged in water which is effective for heating the water to generate steam which is discharged from the pressure vessel for use in powering a steam turbine-generator for producing electrical power, for example.
Typically surrounding the pressure vessel within the containment vessel is an annular suppression pool or wetwell which serves various functions including being a heat sink during postulated accidents. For example, one type of accident designed for is a loss-of-coolant accident (LOCA) in which steam from the pressure vessel leaks therefrom into the drywell. Following the LOCA, therefore, the reactor is shut down but residual decay heat continues to be generated for a certain time following the shutdown. In one conventional safety system, the pressure vessel is depressurized by discharging the steam into the wetwell for cooling and condensing and for preventing unacceptably large pressure increases within the containment vessel itself. Traditional safety-related equipment typically require AC power for effective operation.
Accordingly, improved nuclear reactor plants are being developed to reduce or eliminate the need for AC power safety systems following a LOCA, for example. In one design designated a Simplified Boiling Water Reactor (SBWR), a Passive Containment Cooling System (PCCS) is provided for removing heat from the containment vessel during the LOCA. One example of a PCCS is disclosed in U.S. Pat. No. 5,059,385--Gluntz et al, assigned to the present assignee, wherein the wetwell, or suppression pool, is enclosed and separated from the drywell within the containment vessel, and a Gravity Driven Cooling System (GDCS) pool is located above the wetwell within the containment vessel and is vented to the drywell. An isolation pool is disposed above the GDCS pool and contains an isolation condenser having an inlet disposed in flow communication with the drywell, and an outlet joined to a collector chamber from which a vent pipe extends into the wetwell and a condensate return conduit extends into the GDCS pool. The isolation condenser provides passive heat removal from the containment drywell following the LOCA, with steam released into the drywell flowing through the inlet into the isolation condenser wherein it is condensed. The non-condensable gas within the drywell, such as nitrogen, is carried by the steam into the isolation condenser and must be separated therefrom to provide effective operation of the isolation condenser. The collector chamber separates the non-condensable gas from the condensate, with the separated non-condensable gas being vented into the wetwell, and the condensate being channeled into the GDCS pool.
This system relies on the pressure different between the drywell and the wetwell, and, therefore, a water trap is provided at the end of the condensate return conduit in the GDCS pool to restrict backflow of heated fluids from the containment vessel to the wetwell via the condensate return conduit which would bypass the isolation condenser.
Accordingly, this system is configured to transport the non-condensable gas from the drywell to the wetwell and then condense steam from the drywell in the isolation condenser. The non-condensable gas will remain in the enclosed wetwell until the condenser condenses steam faster than it is released from the pressure vessel. When this occurs, the isolation condenser is effective for lowering the drywell pressure below that of the wetwell which will cause conventional vacuum breakers joined to the wetwell to open, and allow the non-condensable gas stored in the wetwell to return to the drywell. However, these gases may then again be channeled into the isolation condenser and lower the cooling effectiveness thereof until the steam release in the drywell again increases the pressure thereof above that of the wetwell which will cause the vacuum breakers to close and the cycle to repeat with the non-condensable gas again being vented into the wetwell from the isolation condenser.
By continually returning the non-condensable gas to the wetwell, the overall containment pressure remains relatively high especially in the enclosed wetwell itself, which must be suitably accommodated by providing stronger, and therefore more expensive, containment walls for example. Furthermore, incremental heating of the top layer of the wetwell pool each time the isolation condenser vents the non-condensable gas therein, and any minor leakage from the vacuum breakers themselves may also cause the containment pressure to slowly rise.